Why is it bad to implicitly commit a Unit of Work in a Dispose() method?

Question

I wrote a UnitOfWork implementation that does not expose a public Commit() method. Instead the UnitOfWork implements IDisposable and the Commit is executed in the Dispose() method. I don't see any immediate problems with this, but it seems unorthodox so I want to know if you guys can point out some major reason not to do this that I am overlooking.

Here is some sample code:

public class DataService
{
    public DataService()
    {
        _db = new MyDataContext();
        _exceptionHandler = new SqlExceptionHandler();
    }
    private readonly MyDataContext _db;
    private readonly SqlExceptionHandler _exceptionHandler;
    public void Add(Product product, Cart cart)
    {
        using(UnitOfWork unitOfWork = new UnitOfWork(_db, ex=>_exceptionHandler.Handle(ex)))
        {
            unitOfWork.Create<CartItem>(new CartItem{CartId = cart.Id, ProductId = product.Id});
            unitOfWork.Update<Product>(x => x.Id == product.Id, product => { product.OrderCount++; });
        }
    }
}


public class UnitOfWork : IDisposable
{
    private readonly DataContext _dataContext;
    private readonly Func<Exception, bool> _handleException;
    private bool _dirty;

    public UnitOfWork(DataContext dataContext, Func<Exception,bool> handleException)
    {
        _dataContext = dataContext;
        _handleException = handleException;
    }

    private Table<T> Table<T>()
        where T: class
    {
        return _dataContext.GetTable<T>();
    }
    private T[] Find<T>(Expression<Func<T,bool>> select)
        where T: class
    {
        return Table<T>().Where(select).ToArray();
    }

    public void Create<T>(T persistentObject)
        where T: class 
    {
        Table<T>().InsertOnSubmit(persistentObject);
        _dirty = true;
    }
    public void Update<T>(Expression<Func<T, bool>> select, Action<T> update)
        where T : class
    {
        var items = Find<T>(select);
        if (items.Length > 0)
        {
            foreach (var target in items) update(target);
            _dirty = true;
        }
    }
    public void Delete<T>(Expression<Func<T, bool>> select)
        where T : class 
    {
        var items = Find<T>(select);
        switch (items.Length)
        {
            case 0: return;
            case 1:
                Table<T>().DeleteOnSubmit(items[0]);
                break;
            default:
                Table<T>().DeleteAllOnSubmit(items);
                break;
        }
        _dirty = true;
    }

    public void Dispose()
    {
        if (_dirty)
        {
            Commit(1);
        }
    }

    private void Commit(int attempt)
    {
            try
            {
                _dataContext.SubmitChanges();
            }
            catch (Exception exception)
            {
                if (attempt == 1 && _handleException != null && _handleException(exception))
                {
                    Commit(2);
                }
                else
                {
                    throw;
                }
            }
    }
}  

Answer 1

Because an unhandled exception will commit the transaction. And an exception implies that something did not go as planned = transaction should not be committed.

It's far better to Rollback in Dispose if Commit has not been called before disposing.